Most common cause of hypothyroidism in iodine-sufficienct regions.An autoimmune disorder with antithyroid peroxidase (antimicrosomal) and antithyroglobulin antibodies.- Associated with HLA-DR3, DR5.- ↑ risk of non-Hodgkin lymphoma (typically of B-cell origin).

May be hyperthyroid early in course due to thyrotoxicosis during follicular rupture.

Histologic findings: Residual follicles surrounded by Hürthle cells (large oxyphilic cells filled with granular cytoplasm), mononuclear infiltrate consisting of lymphocytes and plasma cells with germinal centers.

Findings: diffusely moderately enlarged, nontender thyroid.